Cryogenic structural insulating panels



June 13, i967 J. KoHN ETAL 3,325,037

CRYOGENIC STRUCTURAL INSULATING FANELS Filed NOV. l2. 1963 OOOOOGOQOO IN VENTORS E deA/v KOH/yA 0.9 -L BY JACQUES KOH/Y l" z" 3" 4" e" w 54am Www/wss O hl- OOOOOOOOOOOOQj- W COLD United States Patent 3,325,037 CRYGGENHC STRUCTURAL INSULATING PANELS .lean Kohn, 46 E. 91st St., New York, NY. 10028, and ttes Kohn, 2440 Sedgwick Ave., Bronx, NX.

Filed Nov. 12, 1963, Ser. No. 322,849 2 Claims. (Cl. 220-9) This invention relates generally to cryogenic enclosures lined with thermal insulation, and more particularly to an improved cryogenic sandwich structure formed of an endgrain balsa core laminated to facing sheets, one of which is subjected to extremely low temperatures, while the other is exposed to ambient temperature.

Cryogenics, which deals with the phenomena of extreme cold, is assuming considerable commercial significance. In recent years, for example, liquid gases having low boiling points, such as nitrogen, have been widely used to freeze perishables which are then protectively stored in insulated containers for prolonged periods without spoilage, or are transported by railroad, truck or vessel over long distances without the need for mechanical refrigeration.

It is also the current practice to liquefy natural gass or methane and to carry the gas in the -liquid state in thermally insulated tanks. The fact -that natural gas in liquefied form occupies a volume which is only one six-hundredth of the fuel in the gaseous state, renders the liquefaction process economically feasible even when the liquid must be transported for thousands of miles from the oil Well, where it is available as a by-product, to the consumer market. To this end, ocean-going vessels have been specifically fitted to carry cargoes of liquefied natural gas.

The primary concern of the present invention is with containers intended for cryogenic purposes, wherein the load, which may be in liquid or solid form, is at an extremely low temperature and must therefore be thermally insulated from ambient tempreature. By ambient temperature is meant the temperature of the ambient air or water to which the loaded container is exposed in storage or transit. By cryogenic container is meant any form of thermally-insulated, low-temperature enclosure, whether a crate or box, intended for rail transportation, a thermally insulated trailer truck, or an insulated tank to be installed on a barge or vessel.

In all forms of cryogenic containers, the structural and thermal problems are similar, for the container must be of sufficient structural strength to support the load under the most severe conditions encountered in transit, and yet the insulation must be such as to maintain the low ternperature of the load within the proper limits despite wide variations in ambient temperature. Moreover, the thermally-insulated structure must be capable of withstanding the stresses produced by the wide temperature differential 'between the cryogenic load temperature and ambient temperature.

The extremes of temperature to which the cryogenic container is subjected will be appreciated when it is realized that cold liquid hydrocarbons at atmospheric pressure have a temperature in the order of *'25 8 F., whereas ambient temperature may range between 0 F. and +1159 F. In the case of liquid nitrogen, the cryogenic temperature is even lower.

It has heretofore been known to construct cryogenic containers using 'balsa as the insulating liner. But while balsa has inherent mechanical and thermal properties which render it suitable for this purpose, containers of the prior-art type have not taken full advantage of balsa and in fact, have employed this material under conditions giv- `ing rise to serious practical drawbacks.

For example, on the generally accepted assumption that flat-grain balsa has thermal characteristics which are dis- ICC tinctly superior to end-grain balsa, it has been the prior practice to line cryogenic containers with flat-grain balsa despite the inferior mechanical properties of this material. In end-grain balsa, the load is imposed in a direction parallel to the grain, hence it has a far higher compressive strength Ithan flat-grain balsa where the load is perpendicular to the gain.

Moreover, in conventional cryogenic containers of the type using flat-grain balsa, the uneven shrinka-ge characteristics of this material made it necessary to incorporate expansion joints in the insulation panels to allow for warpage under cryogenic conditions. Apart vfrom the greater cost and complexity in design created by such joints, they have the further disadvantage of introducing thermal bridges of relatively high thermal conductivity.

Accordingly, it is the principal object of the present invention to provide a cryogenic `container which is lined with insulating sandwich panels having an end-grain balsa core, the structure exploiting the optimum structural and thermal characteristics of this material.

More specifically, it is an object of this invention to provide a cryogenic insulating panel having an end-grain balsa core, one surface of which is laminated to an inner facing sheet capable of withstanding exposure to cryogenic temperatures, the other surface of which is laminated to an outer facing sheet capable of withstanding exposure to ambient temperature. A salient feature of a laminated sandwich panel in accordance with the invention is that it has a uniform coeicient of thermal expansion or contraction and thereby obviates the need for expansion joints and other expedients which are necessary when panels tend to warp under cryogenic conditions.

Also an object of the invention is to provide a cryogenic container incorporating panels of the above-described type, which containers are highly efficient both structurally and thermally, and yet are of simple, low-cost design.

For a better understanding of the invention, as well as other objects and further features therof, reference `is made to the following detailed description to be read in conjunction with the annexed drawing, wherein:

FIG. 1 is a transverse section taken through a cryogenic container incorporating laminated insulating panels in accordance with the invention;

FIG. 2 is a perspective view of one of the laminated panels;

F IG. 3 is an enlarged section taken through the insulated panel showing how a series of panels is interconnected;

FIG. 4 schematically shows in exaggerated form, the balsa grain of the panel under conditions where the temperature on both faces is about the same;

FIG. 5 schematically shows in exaggerated .form the balsa grain of the panel under conditions where an extreme temperature differential exists between the faces; :and

FIG. 6 is a graph showing the comparison between U,- factor values of end-grain and fiat-grain panels.

Referring now to the drawings, and more particularly to FIG. 1, the basic structure of a container in accordance with the invention is shown in the form of a cargo vessel having a hull 10 and an internal reinforcing frame 11 therefor, the load 12 in the form of liquefied natural gas being shipped in an inner tank 13 supported within the hold of the vessel by insulating panels 14 which are interposed between the hull frame and the tank and surround the tank to maintain the cold temperature of the liquid.

The cargo container shown herein by way of illustration only, and the hull of the ship represents the outer skin of the cryogenic container, which in the case -of a shipping crate coul-d be a thin aluminum shell. While for shipping liquid methane, the inner tank 13 is preferably of nickel alloy steel or aluminum, it is to ybe understood that in other instances the tank may be dispensed with entirely, as in transporting a solid load frozen by liquid nitrogen, The significant aspect of the invention resides in the nature of the insulating panels 14, for these panels must -be able to withstand the mechanical forces imposed thereon by the load in the course of transit, as well as the internal stresses created by the temperature differential. At the same time, they must MECHANICAL PROPERTIES OF BELCOBALSA [Data for pieces averaging 12% moisture content] Weight in pounds per eubie foot Specific Gravity 0962 176 24S Compressive Strength (pounds per square inch):

(A) Parallel to grain (end grain):

Stress at proportional limit 500 1, 450 2, 310 Maximum crushing strength-- 750 1, 910 2, 950 Modulus of Elasticity 330, 000 76S, 000 1, 164, 000 (B) Perpendicular to grain (fiat grain):

Stress at proportional limit:

High Strength value. 84 144 198 Low Strength value. 50 100 145 Modulus of Elastieity:

High Strength value... 16,000 37,000 55,000 Low Strength value 5,100 13,000 19, 900 Tensile Strength (pounds per square inch): (A) Parallel to grain (end grain)-Maximurn. 1,375 3,050 4, 52o (B) Perpendieular to grain (flat grain):

Maximum high strength value.. 112 170 223 Low strength value 72 118 156 Hardness (pounds)load required to embed ball to le its diameter:

(A) Parallel to grain (end grain) 102 250 386 (B) Perpendieular to grain (fiat grain):

High Strength Value 50 120 186 Low Strength Value 47 103 151 afford effective thermal insulation without thermal bridges which impair the advantages obtained from the extremely low thermal conductivity of balsa.

As shown separately in FIGS. 2 and 3, the panels 14 are lconstituted `by a core of end-grain balsa wood 15, one surface of which is laminated by an adhesive layer 16 to an inner facing plate 17, exposed to cryogenic temperatures, while the other surface is laminated by an adhesive layer 18 to an outer facing plate which is exposed to ambient temperature. The cryogenic temperature in the example of FIG. 1 is the temperature of liquid methane, whereas ambient temperature is that of the water with respect to that portion engaging the submerged areas of the hull, and that of the air with respect to that portion engaging the area of the hull above the water line.

The Ibalsa core represents a major aspect of the invention, hence the nature of this material and its thermal and mechanical properties will be reviewed briefly. Balsa is the lightest commercial wood, and is derived from a fast-growing tree found in Central and South America, which attains maturity within six to seven years. After being converted into lumber, it is kiln-dried and once dried it will not be subject to deterioration by decay, if used properly. One well-known, high-grade form of commercially available balsa is sold unde-r the trademark Belcobalsa, and the mechanicam and thermal values set forth below are for various grades of this material.

Balsa has outstandingproperties unique in the lumber field. It averages less than nine pounds per cubic foot, which is of the weight of the lightest North American species. Balsas cell structure affords a combination of high rigidity and compressive and tensile strength far superior to any composite, matted or synthetic material. Because of its inconspicuous growth increments and lightness, -balsa is dimensionally stable and it may be processed by standard woodworking techniques.

It is known that end-grain balsa is capable of supporting far greater loads than flat-grain material of the same density, and also that low-density material will in the end- Despite the mechanical advantages of end-grain balsa, it has heretofore been the practice to use flat-grain 'balsa in situations where effective thermal insulation is called for, on the principle that flat-grain is more efficient thermally than end-grain. This selection was apparently based on compartive k-factor values of end-grain and flatgrain balsa. k-factor is the symbol for thermal conductivity which is the amount of heat, expressed in B.t.u., transmitted in one hour through one square foot of homogeneous material one inch thick, for each degree Fahrenheit of temperature difference between opposing surfaces of the material.

But for the same average densities, the k-factor for end-grain balsa has always been thought to be higher. For example, in the Wood Handbook, published by the U.S. Department of Agriculture (Publication No. 72, 1955, page 46) it is stated, In normal Wood the relative rate of heat flow is approximately the same in the radial and tangential directions, but thermal conductivity is generally 21A to 2% times faster along the grain than in the transverse direction.

While the above-quoted statement is true for balsa wood without any facings and in thin sections of about 1A thickness, we have discovered that when end-grain balsa is sandwiched between facing sheets and is in a thickness of at least two inches, the U-factor of the sandwich panel, for practical purposes is at least equal to that of an otherwise equivalent panel made of flat-grain balsa, whereas the resultant end-grain sandwich is much superior structurally to that of the fiat-grain panel. In most cryogenic panels, at least four inches of balsa is required for insulation purposes, Thus the thickness of the balsa core is a critical factor with respect to its thermal characteristios.

The U-factor is the over-all heat transmission coeflicient and is determined by the amount of heat expressed in B.t.u. transmitted in one hour through one square foot of panel section for each degree Fahrenheit of temperature difference between air on the facing of the sandwich panel section von the warm side and air on the cold side.

The U-factor takes into account not only the k-factor of the faces and cores respectively, but the thickness of these elements. It has been found that convection effects which are believed to be responsible for the higher thermal conductivity of end-grain balsa, are minimized when the balsa thickness is at least in excess of two inches, because of internal barriers which inhibit thermal flux in such thicknesses. In our studies of the relationship between the U-factor of end-grain sandwich panels as compared to flat-grain, we have arrived at the following U- factor ratios for end grain/flat grain:

Thus for sandwich panels of four-inch thickness, the U-factor ratio of end-grain to flatgrain is equal, and for a greater thickness, the ratio distinctly favors end-grain. The results of this study are shown graphically in FIG. 6, where curve A is the U-factor comparison for bare balsa and curve B is the comparison for balsa in an aluminumcovered sandwich. It will be appreciated that these re sults are contrary to what one would be led to expect on the basis of the known k-factor values for end and flat grain.

It will be evident, therefore, that in lining a container, such as that shown in FIG. 1, with sandwich panels 14 having a balsa core 15 oriented in the end-grain direction and laminated to facing sheets 17 and 18, the lining will exploit optimum properties of the balsa from both the thermal and mechanical standpoints.

It is also to be borne in mind that balsa panels are particularly advantageous in cryogenic applications, in that its thermal resistivity increases as a straight-line function with a reduction in temperature, and not only does the U-factor of the sandwich panel improve at extremely low temperatures, but its physical properties also improve as the temperature drops. Furthermore, the natural resilience of balsa is of value in those container situations where the panels are subjected continually to bending forces, as is the case on vessels in constant movement. Balsa can be bent repeatedly without evidencing fatigue, whereas synthetic insulation tends to break down in such circumstances.

It is important to the present invention that the inner facing sheet 17 which is subjected to the cryogenic temperatures be of a material adapted to function effectively in this range, and that the outer facing sheet 19 have properties suitable to the ambient temperature to which it is exposed, and that both sheets be securely and permanently bonded to the balsa.

Suitable inner and outer facings 17 and 19 can be made of metal, plastic or wood, it being important that the nature of the facing be appropriate to the temperatures involves and that the mechanical properties of the facings in tension or compression in the direction perpendicular to the grain of the balsa, be superior to the mechanical properties of balsa in tension or compression across the grain. In practice, the inner facing may be made of aluminum or stainless steel, with the outer facings of wood or plywood. The adherent layers 16 and 18 must be suitable for cryogenic purposes, such as epoxies, Vinyl phenolics and polyurethane elastomers.

As shown in FIG. 3, where a large container surface is to be lined with end-grain balsa sandwich panels in according with the invention, it may be necessary to form the lining of connected sections, such as sections X and Y. In order to join sections X and Y and to provide an effective seal therebetween, the adjacent edges of sections X and Y are preferably stepped in the manner shown and are joined together by a T-shaped coupling plug which is seated on the steps to provide a coupling therebetween, the plug being bonded in place by suitable adhesives. Alternatively, the edges of the panels may be straight-edged and sealed at the juncture thereof.

The absence of expansion joints is to be noted, for as pointed out previously, flat-grain balsa tends to warp when subjected to cryogenic temperatures and such joints are necessary to allow for distortion of the panels, This drawback is not encountered in end-grain panels for reasons which will now be analyzed.

The coefficient of linear expansion of balsa is:

Tangential-8.6 l0*6 inch per inch per degree F. Radial-6.92X106 inch per inch per degree F. Longitudinal-l.99 l0*6 inch per inch per degree F.

FIVE POUNDS/CUBIC FOOT WOOD Average width (microns) of cells Volume percentage of each type of cell Fibers 53. 8 41% Parenchyma 65. 8 56% Vessels 228. 4 3% (Ray cells have not been included since they do not add any strength to an end-grain panel).

The length of these cells is about 550 microns.

The number of cells showing on one square foot of cross-section is about 50 million.

The number of cells (excluding ray cells) in one cubic foot, is about 30 billion.

This myriad structure of minute cells is bonded to the facing sheets, as shown in FIG. 4, where it will be assumed that the temperature on both sides is ambient. The cells, while all bonded to the facing sheets, are effectively independent of each other as far as expansion and contraction are concerned. When, as shown in FIG. 5, one of the facing sheets contracts with cold, the cells follow and actually separate one from the other. But structurally, the end-grain panel does not warp, for each cell is cornparable to an independent column, the columns drawing uniformly closer together with contraction of the facing sheets or uniformly apart with expansion thereof.

Thus the end-grain balsa panels exhibit a uniform coeflicient of linear expansion which precludes the need for expansion joints and similar expedients. At the same time, the panels, although of light weight, are structurally very strong, and it is possible, therefore, to build a cryogenic container with a relatively weak outer shell and without reinforcing ribs, relying on the panels to impart the necessary strength to the container.

In a preferred form of the sandwich structure, particularly when used in conjunction with a tank containing liquid methane, instead of a single layer of end-grain balsa between the facing sheets, two, three or more layers of balsa are used, the layers being bonded together with a material such as Teflon, polyurethane, elastomer or other material forming a leakage-tight secondary barrier. The secondary barrier is impervious to gas vapor, the methane tank serving as the primary barrier. Thus in the event of a failure in the primary barrier, the secondary barrier serves to prevent flow of methane through the insulation panel, which could be dangerous when the tank is held within a ship. The secondary barrier is effectively a film and it would be applied as a liquid resin, which when dried provides the desired bond Vand impervious barrier.

While there has been shown a preferred embodiment of cryogenic enclosure in accordance with the invention, it will be appreciated that many changes and modications may be made therein, without, however, departing from the essential spirit of the invention as defined in the annexed claims. For example, in some container constructions, it is feasible to use the inner facing of the sandwich panels as the interior enclosure to accommodate the cryogenic load, without the need for an inner tank or equivalent means. It is intended, therefore, to cover in the claims all such changes as fall within the true spirit of the invention.

What is claimed is:

1. In a vessel for shipping liquid methane and having a hull, a tank within said hull for accommodatingV liquid methane and constituting a primary barrier between said methane and said hull, and a thermal insulating structure surrounding said tank and supporting the methane load within the hull, said structure being constituted by sandwich panels of exceptional structural strength each of which includes a plurality of balsa layers all disposed with their grain extending in a direction perpendicular to the -faces of the panels to form a multi-layer core, the adjacent faces of said layers being interbonded with a iilrn of methane-impervious material forming a secondary barrier, and facing sheets laminated to the upper and lower lfaces of `the multi-layer core, said multi-layer core having a thickness in excess of that it which the resultant U factor of the sandwich panel is at least equal to that of a sandwich panel employing a fiat grain balsa core of the same thickness.

2. A cryogenic container for accommodating and sup- 8 porting a load whose temperaturey is in the cryogenic region and for thermally insulating said load from ambient temperature, said container being lined with a plurality of sandwich panels, each panel comprising:

(a) a pluraiity of balsa layers all disposed with the grain direction extending perpendicular to the faces of the panel,

(b) a plurality of films of synthetic plastic material impervious to the vapors from the contents of the container interbonding said layers to form an integrated multi-layer core, and (c) facing sheets laminated to the upper and lower faces of the multi-layer core to define the sandwich panel, said multi-layer core having a thickness exceeding that at which the resultant U factor of the panel is at least equal to that of a sandwich panel employing a flat-grain balsa core of the same thickness.

References Cited UNITED STATES PATENTS 2,280,094 4/ 1942 Madsen. 2,479,342 8/ 1949 Gibbons et al. 2,562,976 8/1951 Winnick. 2,859,895 11/1958 Beckwith. 3,019,937 2/1962 Morrison 220--65 3,112,043 11/1963 Tucker 220--10 3,150,793 9/1964 Messer 220-9 FOREIGN PATENTS 932,581 7/1963 Great Britain.

THERON E. CONDON, Przmmy Examiner.

JAMES R. GARRE'IT, LOUIS G. MANCENE,

Examiners 

1. IN A VESSEL FOR SHIPPING LIQUID METHANE AND HAVING A HULL, A TANK WITHIN SAID HULL FOR ACCOMMODATING LIQUID METHANE AND CONSTITUTING A PRIMARY BARRIER BETWEEN SAID METHANE AND SAID HULL, AND A THERMAL INSULATING STRUCTURE SURROUNDING SAID TANK AND SUPPORTING THE METHANE LOAD WITHIN THE HULL, SAID STRUCTURE BEING CONSTITUTED BY SANDWICH PANELS OF EXCEPTIONAL STRUCTURAL STRENGTH EACH OF WHICH INCLUDES A PLURALITY OF BALSA LAYERS ALL DISPOSED WITH THEIR GRAIN EXTENDING IN A DIRECTION PERPENDICULAR TO THE FACES OF THE PANELS TO FORM A MULTI-LAYER CORE, THE ADJACENT FACES OF SAID LAYERS BEING INTERBONDED WITH A FILM OF METHANE-IMPERVIOUS MATERIAL FORMING A SECONDARY BARRIER, AND FACING SHEETS LAMINATED TO THE UPPER AND LOWER 